Currently, a circuit structure shown in FIG. 1 is generally adopted to output a constant current or a constant voltage in the application of a resonant converter. The controlling principle of the circuit structure is as follow. An output current value Io or an output voltage value Vo of a secondary circuit is sampled, and a feedback signal is outputted to a control unit via an optical isolator. A switching frequency of a switch is controlled by the control unit according to the feedback signal, such that the resonant converter is controlled to output a constant current or a constant voltage.
In the conventional technology as shown in FIG. 1, since a controlling is performed by sampling the current value or the voltage value of the secondary circuit, the optical isolator is inevitable to isolate the secondary circuit form the control unit, thereby resulting in various components and a complicated circuit. Furthermore, the optical isolator is expensive and has a limited lifetime, which impact a lifetime of the circuit, thereby being unfavorable for applying the resonant converter.